The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. The article to be coated is the cathode in the electrical circuit with the counter-electrode being the anode.
Resin compositions used in cathodic electrodeposition baths are also well known in the art. These resins are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen The nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslinking agent and then salted with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives at the coating site to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and so forth.
The coated object is removed from the bath after a set amount of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinking.
The prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307.
Some important characteristics of the electrocoat primer to the automotive industry are chip resistance and intercoat adhesion of the electrocoat primer. Chip resistance is important to the automotive industry because automotive coatings are inherently subject to chipping from pebbles, debris and so forth. Chip resistant primers can help defray warranty costs associated with the coating. It is more difficult to obtain satisfactory chip resistance in an underbake condition. (Underbake is the lower limit of the cure range for a given system. Underbake for a typical current commercial cathodic electrocoat system is about 20.degree. F. to 30.degree. F. below the standard bake temperature for a given time.) In the automotive industry underbake is especially prevalent on the front grill section of the car because of the thicker and heavier metal. Often the front grill metal never reaches standard bake temperature and thus is underbaked. Unfortunately, this is the area of the car which most needs chip protection because of its location.
It is well known in the art that adding additional amounts of cure catalyst will help the cure at underbake conditions and give better chip resistance. The trade off however, is that the excess cure catalyst often causes overcure (brittleness) at standard or overbake conditions because of higher crosslinking density. (Overbake is the upper limit of the cure range for a given system. Overbake for a typical current commercial cathodic electrocoat system is about 20.degree. F. to 30.degree. F. higher than standard bake temperature for a given time.)
Intercoat adhesion likewise is important because the electrocoat primer is typically covered with a topcoat. If the topcoat is damaged, good intercoat adhesion will prevent the flaw from expanding.
What is needed is a cathodic electrocoat primer which gives improved chip resistance at underbake without sacrificing standard or overbake chip resistance and also gives improved intercoat adhesion.